788 DR ALEXANDER BRUCE AND DR JAMES W. DAWSON ON 
(a) In regeneration.—It has frequently been pointed out that a neurogenous tissue, © 
with many of the characters of embryonic nerve tissue, arises in the distal end of a 
severed nerve as a result of the activity of the cells of Schwann’s sheath. DuRants, 
BaLuance and Stewart, and Berne have shown that in the specific tissue a differ- 
entiation to axis-cylinder and incomplete myelination may take place even when the 
distal end is not united to the central end. For a complete differentiation of axial 
fibrils and myelin, all admit that the influence of the central neuroblast is essential. 
(b) In tumour formation—The generalisation that the genesis of nerve fibres in 
regeneration recapitulates the stages of its first development has necessarily its limita- 
tions, and one of these may be the modifications imposed on the cells derived from the 
proliferation of the sheath of Schwann nuclei. It is not necessary, then, to deny the 
possibility that in tumour formation peripheral neuroblasts can form completely 
differentiated fibres, for here we are dealing with cells left undeveloped in the tissue. 
The evidence from the cases of ganglio-neuroma already mentioned leads us to suppose 
that the nerve fibre cells or their precursors differentiate to the development of nerve 
fibres which show many of the characters of fully formed nerves, stopping short again 
of the stage of complete axial fibril and myelin differentiation. 
From this evidence, therefore, we gather that it is quite conceivable that the 
fusiform cells in our preparations have evolved to the formation of nucleated tubes in 
the condition of the protoplasmic bands of Durante in the medulla and pons, and in 
the cord to a yet completer stage of differentiation. | LENHOSSEK, a convinced 
centralist, states in his latest paper that he cannot deny the evidence that the sheath 
of Schwann cells (lemnoblasts) may in pathological conditions, in virtue of their origin 
from the neural crest, produce true nerve fibres. To LennosseK the sheath of Schwann 
cells are the glia elements of the peripheral nervous system, but he thinks that, in the 
uncertainty of our conception of the actual manner of cell differentiation, it is possible 
that a mother cell which should have differentiated along one line may in pathological 
conditions differentiate along another which had the same histogenesis. 
This attempt at a possible interpretation and correlation of our observations is not 
regarded as in any sense a logical proof. It is not contended that the facts prove the 
truth of this conception, but it is maintained that this view, though based only on 
deductions, gives clearness to an otherwise quite unintelligible process. 
It is convenient at this stage to consider one or two criticisms of the peripherist 
standpoint with special references to appearances in our preparations. 
The necessity of the influence of the central ganglion cell to complete the 
differentiation of the new nerve fibre, arisen from the proliferation of the sheath of 
Schwann cells, in regeneration has seemed an unassailable areument in favour of the 
centralists’ view. To this criticism the reply has been made that if it is true that 
every cell differentiates in view of a function, it is necessary to remember that it is the 
functioning which determines and perfects the cell differentiation. The nerve paths 
in the embryo remain as embryonic nerves till the function of the tract is called into 
